5 '-phosphoramidates and 5 '-diphosphates of 2 '-O-allyl-beta-D-arabinofuranosyluracil, -cytosine, and -adenine: Inhibition of ribonucleotide reductase

Continuing our studies on ribonucleotide reductase (RNR) mechanism-based in hibitors, we have now prepared the diphosphates (DP) of 2'-O-allyl-1-beta-D -arabinofuranosyl-uracil and -cytosine and 2'-O-allyl-9-beta-D-arabinofuran osyl-adenine and evaluated their inhibitory activity against recombinant mu rine RNR. 2'-O-Allyl-araUDP proved to be inhibitory to RNR at an IC50 of 10 0 mu M, whereas 2'-O-allyl-araCDP was only marginally active (IC50 1 mM) an d 2'-O-allyl-araADP was completely inactive. The susceptibility of the pare nt nucleosides to phosphorylation by thymidine kinase and 2'-deoxycytidine kinase was also investigated, and all nucleosides proved to be poor substra tes for the above-cited kinases. Moreover, prodrugs of 2'-O-allyl-araU and -araC monophosphates, namely 2'-O-allyl-5'-(phenylethoxy-L-alanyl phosphate )-araU and -araC, were prepared and tested against tumor cell proliferation but proved to be inactive. A molecular modeling study has been conducted i n order to explain our results. The data confirm that for both the natural and analogue nucleoside diphosphates, the principal determinant interaction with the active site of RNR is with the diphosphate group, which forms str ong hydrogen bonds with Glu623, Thr624, Ser625, and Thr209. Our findings in dicate that the poor phosphorylation may represent an explanation for the l ack of marked in vitro cytostatic activity of the test compounds.